Electret microphones or silicon microphone modules are being employed with increasing frequency in electrical devices used for communications between several parties, such as, for example, microphones. The silicon microphone modules here may be manufactured easily in a mass-production process of a semiconductor company. This allows manufacturing silicon microphone modules in great numbers at low manufacturing costs.
The silicon microphone modules or electret microphones typically comprise a passive voltage supply across resistors. Thus, the resistance must be selected to be very high so that the value of the resistor used will be in a range from 10 GΩ to several 10 GΩ, since a combination or connection of the microphone capacitance and the resistor is to produce or form a high pass having a very low cutoff frequency for an output signal of the sensor circuit.
At the same time, the electrical high pass for the output signal of the sensor circuit acts as a low pass for a disturbing noise, such as, for example, thermal noise of the resistor. If the cutoff frequency is far below the frequency of the useful band, the disturbing noise will be filtered out, the result being an improvement in the signal-to-noise distance. The resistor used should also for this reason have a high value.
The resistor in the series connection comprising the microphone which is used for the voltage supply of the microphone has been implemented so far as an external device or an integrated polysilicon resistor. However, both resistor implementations entail disadvantages. The resistor implementation as an external device causes additional manufacturing costs which, for example, result from the fact that the additional component of the external resistor is to be arranged on a board on which the microphone is arranged. At the same time, further stocking of an additional component, i.e. the external resistor, produces additional logistic expenses for stocking this additional external component for a manufacturer of devices employing the microphones interconnected with the external resistors. The additional logistic expenses produce additional expenses and thus costs for the manufacturer of a device comprising such a voltage supply.
At the same time, an implementation of the resistor used in the voltage supply of the microphone as an internal resistor, exemplarily on a chip, in the form of a lightly doped polysilicon which is used as a resistor material is flawed by considerable disadvantages. In order to produce a resistor having such a high total resistance, a high number of resistor squares are to be implemented on the chip, entailing considerable chip area consumption. At the same time, a series resistor implemented in this way typically having such a high resistance is technologically difficult to manufacture and, above all, very difficult to reproduce in mass production. The result is that in mass production considerable variation of the series resistor value in microphone devices produced in this way result from variations of the process parameters so that a considerable portion of the microphone devices produced in this way does not have the specified values for the series resistor and must be discarded after a microphone device test. Due to the high portion of microphone devices to be discarded, the manufacturing costs for the microphone devices comprising a lightly doped polysilicon resistor as a series resistor are increased.
An additional disadvantage of the series resistor implementation as a polysilicon resistor is that such a high-resistance polyresistor and/or polysilicon resistor has a strong temperature trend. This is of particular disadvantage in mobile phones used both at low temperatures in winter and with strong solar irradiation, since the microphone devices implemented therein having a polysilicon resistor which is used for a voltage supply have a strong temperature trend, the result being that the electrical performance of the microphone and thus the sensitivity of the sensor implemented in this way for receiving sound vary strongly in dependence on the temperature.